The invention relates to an exposure control process for regulating the density in zones of photographic copies of a copy master scanned by areas. Further, the invention relates to a photographic copying apparatus for the production of photographic copies of copy masters with a measuring station.
The production of high quality copies (e.g., positive images) of copy masters (e.g., negatives or slides) imposes high requirements with regard to the occupational skills of the operating personnel in the case of manual exposures and with regard to the measuring and evaluating devices and the exposure control process in automatic photographic equipment. In known exposure control devices of photographic copying equipment, the copy masters are scanned and measured in sections. From the measured values density and color correction values are determined, together with necessary quantities of copying light. In an exposing station, the photographic copies are then prepared by exposing the copy material to the quantities of copying light determined. In this manner the copy master is exposed overall and at every location to a copying light of the same intensity.
This integral exposure process prohibits individual variation of the copies to be produced, either to obtain special effects or more importantly to correct inadequate exposed copy masters or masters with an excessive contrast range. In the preparation of photographic copies, copy masters with overexposed or underexposed zones are frequently encountered. With this known integral exposure process, photographic copies with overexposed or underexposed areas are often obtained from such masters, in which details relevant to the image disappear. Any correction of the quantity of copy light always affects the entire copy. For example, a reduction of the quantity of copy light brightens otherwise dark copies so that details become visible. However, correctly exposed areas are reproduced with excessive brightness, which again leads to a loss of information in these areas, that is in most cases important for the image. Conversely, increasing the quantity of the copy light to render details visible in zones that are otherwise too bright results in darkening of correctly exposed areas.
It is known from EP-A-315 589 that in manual exposure processes, density masks are held in front of a copy master in the beam path of the copy light in order to compensate for over- or underexposed areas in the photographic copy. In these processes, when amateur or professional photographers improvise density masks from film rests or other materials, correct arrangement of the density masks in the beam path of the copy light is very time consuming and hardly reproducible. Furthermore, this form of locally modifying the density may be examined for correctness only after the preparation of the photographic copy, so that copies which overall appear to be correct are frequently obtained only after several experiments. For this reason, it is proposed in EP-A-315 589 and also in DE-A-2 820 965, to carry out density corrections by areas using an opto-electric mask, which is located in front of the copy master in the beam path of the copy light. In particular, these two documents describe masks of liquid crystal elements arranged as a matrix, the transparency of which may be varied individually by electric control signals. To control and monitor the local density modified in this manner, a beam splitter element is placed between the copy master and the copy material, whereby part of the light coming from the mask and the copy master is diverted and conducted to a video camera together with an image processing unit. The processed image inverted to the positive may then be observed and corrected, if necessary.
While liquid crystal masks permit local density modifications to be carried out in a convenient manner, such masks do have disadvantages. Liquid crystals have a relatively low long term stability. They are also relatively sensitive to temperature. If it is realized that in the continuous operation of a photographic copy apparatus temperatures of considerable magnitude appear in the exposure station, it is readily seen that an additional effort must be made to cool the liquid crystal mask in order to prevent a premature overall or partial degradation of the liquid crystal mask. A particularly important disadvantage of liquid crystal masks is their lack of color fastness. This is intended to signify that particularly in the transition from areas of high transmission to areas of lower transmission of the mask, color tints may appear which naturally would have an extremely negative affect on the color impression of the copies to be produced. Another disadvantage of liquid crystal masks is that the matrix like layout of the liquid crystal elements cannot be changed. Consequently, the arrangement of the matrix elements of the liquid crystal mask and the arrangement of the scanning points of the copy master frequently do not coincide. Furthermore, the entire concept of the local modification of the density of the copies to be produced with the aid of liquid crystal masks requires considerable electrical and electronic elements. The system, aside from the additional cooling devices required, is very expensive and is difficult to incorporate into existing devices. The results of the local modification of density may be observed only by additional, very expensive devices (imaging apparatus, image processor, beam splitter) on a screen. Furthermore, the beam splitting element required in the beam path of the copy light may represent an additional source of error in the exposure.